1. Field of the Invention
This invention relates to the production of minerals from oil shale deposits, and, more particularly, to recovery of nahcolite and hydrocarbon products by in situ retorting of mined, nahcolite-bearing oil shale ore from which a major portion of the nahcolite has been separated.
2. Background of the Invention
Nahcolite is a naturally occurring sodium bicarbonate which is sometimes found in substantial quantities in oil shale deposits. As used herein, the terms "ore" and "oil shale ore" refer to such nahcolite-bearing oil shale, and "oil shale" refers to the fraction that remains after a major portion of the nahcolite has been removed from the ore.
Deposits of oil shale ore have not been utilized to a significant extent as an oil source due to the relatively high cost of mining and recovering the oil, and the environmental considerations involved in such an operation. Oil shale ore formations contain hydrocarbons which exist in the form of kerogen. For all practical purposes, kerogen is immobile within the shale. However, it is well-known in the art that hydrocarbons can be recovered by heating the oil shale in a process called retorting. Two basic techniques have been utilized for this purpose: surface retorting and in situ retorting.
In U.S. Pat. No. 3,821,353 to Weichman, there is disclosed a process for recovering hydrocarbons, aluminum, sodium carbonate and/or nahcolite from oil shale ore. Further, mechanical separation of nahcolite from oil shale, leaching of nahcolite from oil shale, and recovery of alumina and aluminum hydroxide from retorted oil shale are disclosed. In particular, Weichman discloses two methods of retorting, neither of which involves in situ retorting.
Methods of in situ retorting are well-known in the art. As the name suggests, the retort chamber is formed in the oil shale deposit. According to well-known procedures, the retort chamber may comprise one or a plurality of rooms within a gallery. Rooms are formed by removing a portion of the shale from the ore deposit by conventional mining techniques such as room and pillar mining. The surrounding shale is then rubblized by use of explosives, and the rubblized shale is then retorted by in situ combustion or by heating gases externally and passing them through the rubblized bed. With either technique, the hydrocarbons produced are recovered at the lower end of the retort.
In particular, U.S. Pat. No. 3,950,029 to Timmins and 3,957,305 to Peterson describe in situ retorting of oil shale. In the '029 patent, a method of in situ retorting is described wherein a retorting zone is formed in the deposit and the zone comprises at least two galleries separated by a barrier wall which is sufficiently thick to prevent leakage of gas between galleries. Each gallery comprises a plurality of rooms having walls substantially thinner than the barrier wall between galleries. The rooms are constructed by conventional mining techniques of removing a portion of the shale within the defined room and rubblizing the surrounding shale by use of explosives or other suitable techniques. Timmins suggests that one gallery can then be retorted while work continues in an adjoining gallery.
The '305 patent discloses formation of the in situ retort chambers by means of a side excavating machine. A major portion of the excavated shale is deposited in the chambers. Once excavation is completed and the retort chambers are suitably sealed, the chambers formed according to the above description are then retorted.
In situ retorting reduces the problems of cooling and disposing of the spent shale inherent in surface retorting; however, the known and above-described in situ methods likewise have inherent problems which must be considered in making an in situ operation commercially feasible.
Present methods of in situ retorting do not provide for recovery of minerals other than hydrocarbons and, in the instance of hydrocarbon recovery, provisions could be made for more economical, efficient recovery.
In particular, the present methods of retorting which utilize explosives to rubblize the shale have many problems. With blasting it is difficult to control the size distribution of the oil shale ore particles in the retort volume. Many large boulders that result from the blast do not fully retort. Also, the smaller particules, i.e., the "fines," produced by the blast tend to produce areas of low permeability in the retort. Since the burning front in the retorting zone advances more rapidly in the more permeable zones, "channelling" of flow can result during the retorting operation, which can result in substantial quantities of oil shale ore not being fully retorted. Inefficiency and environmental damage also may result from those processes where the in situ retort is not fully sealed. For example, water can leak into the retort during burning, causing great heat loss, and ground water can be contaminated. Finally, methods currently employed for separating nahcolite from oil shale ore can create nahcolite particles which can be carried away by winds resulting in environmentally undesirable dust.